The present invention relates to a nuclear quadrupole resonance (NQR) method and device for detecting a target material using a rotating radio frequency (RF) magnetic field. More particularly, the present invention is directed to an improved NQR surface coil design and method for generating a rotating RF field in a detecting region outside the surface coil.
There are many situations where it is desirable to detect the presence of a target material, that is, a specific substance. For example, detection systems are often used to detect sub-kilogram quantities of narcotics and explosives against a background of more benign materials. Such detection systems are used in airports and other locales to detect these materials, e.g. when hidden in luggage.
NQR is a known technique for detecting a target material. It is an effective means of detecting nitrogenous and chlorine-containing materials, such as explosives and narcotics, owing to the presence of quadrupolar nuclei, e.g. 14N and 35,37Cl, in these substances. This general NQR approach is referred to as xe2x80x98purexe2x80x99 NQR to indicate that no externally applied static magnetic field is required. NQR is generally excited with an RF pulse that induces an NQR signal known as a free induction decay (FID). Trains of pulses induce NQR signals known as spin echoes, related to FIDs. An RF pulse at a specific frequency transmitted to a coil in proximity to a sample of interest induces an NQR signal from the nuclear spins of quadrupole nuclei in these specific materials but not in others. The NQR signal is generally proportional to the quantity of material excited, but the exact magnitude of the NQR signal depends on the relative orientation of the RF field and the molecules of the sample. The direction of the RF field with respect to the sample is, in turn, determined by the geometry of the RF field and the position of the sample relative to the coil.
FIG. 1 is a diagram illustrating a conventional NQR apparatus. A transmitter 20 and a receiver 22 are connected to a probe 24 through a transmit/receive (T/R) switch 26. Probe 24 includes an inductor, such as a solenoid coil 28, forming part of a resonance circuit with other inductors L and capacitors C. To detect the presence of a target material, T/R switch 26 connects transmitter 20 to probe 24 while disconnecting receiver 22 from probe 24. Transmitter 20 then generates and transmits a pulse to probe 24. The pulse is typically formed by an RF signal having a frequency corresponding to the resultant resonance signal of the nuclei of the target material to be detected. Probe 24 receives the pulse, causing coil 28 to store (RF) energy, which in turn produces a corresponding RF magnetic field. When a sample (not illustrated) is positioned near, or inside, coil 28, the RF magnetic field irradiates the sample inducing an NQR signal in a target material.
After subjecting the sample to the RF magnetic field, T/R switch 26 connects receiver 22 to probe 24 while disconnecting transmitter 20 from probe 24. Coil 28 then detects NQR induced in a target material and probe 24 produces a corresponding output signal. The output signal is received and analyzed by receiver 22, thereby establishing the presence of a target material in the sample.
In real world use of NQR for detecting narcotics and explosives, a sample may or may not contain the target material of interest. Due to the selection of a narrow bandwidth of the RF magnetic field for irradiating a sample and because of the large range of NQR frequencies in benign, non-targeted materials, it is unlikely that an NQR signal is induced in such non-targeted materials by the RF magnetic field. Accordingly, an NQR device provides accurate identification of target materials without producing false alarms (false positives).
Unfortunately, a major obstacle to detecting sub-kilogram quantities of these types of materials is the low sensitivity of NQR detection devices. One approach to this problem employs spatially rotating RF magnetic fields generated by two spatially orthogonal RF magnetic fields 90xc2x0 out of phase. In the related fields of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), the use of rotating RF magnetic fields can increase the detected maximum signal by a factor of 2, with a concomitant increase in the signal-to-noise ratio (SNR) of 1.4.
A similar increase in SNR can be obtained for NQR, but for very different physical reasons. For the typical case of a powdered or polycrystalline sample the NQR signal is independent of the direction of the RF field, however only a fraction of the nuclei are excited by the pulse. As described in U.S. Ser. No. 08/904,937, filed Aug. 1, 1997, and incorporated herein by reference, a rotating RF magnetic field can increase the number of nuclei excited by the RF pulse. The detected maximum signal from single pulse excitation increases by a factor of almost 2, with a concomitant increase in the SNR of approximately 1.2. The SNR gain is about 1.4 for spin echo sequences. Also, smaller pulse flip angles may be used to excite the spins, with a corresponding savings in RF transmitter power. Also as set forth in U.S. Ser. No. 08/904,937, the use of rotating RF magnetic fields in NQR can make the detector less sensitive to acoustic and piezoelectric ringing artifacts.
FIG. 2 is a diagram illustrating another NQR apparatus for applying and detecting rotating RF magnetic fields. It operates in a manner similar to the conventional NQR apparatus shown in FIG. 1. A transmitter 30 and a receiver 32 are connected to a probe 34 through T/R switch 36. Probe 34 includes a coil 38, such as a birdcage coil for generating and detecting rotating RF magnetic fields. Transmitter 30 differs from its conventional counterpart 20 in that it provides two RF signals 90xc2x0 out of phase to create the rotating RF magnetic field. Likewise, receiver 32 detects both components of the rotating NQR signal and T/R switch 36 can connect the two transmitter outputs and two receiver inputs to probe 34.
One implementation of a device capable of generating the rotating RF field inside the enclosed coil volume employs what is commonly referred to as a xe2x80x9cbirdcagexe2x80x9d coil geometry. However, some scenarios require a device for excitation and detection outside the coil volume. There are two criteria for such a device: i) the device must generate the two RF magnetic field components substantially orthogonal to each other; and ii) if multiple coils are used, they must have negligible mutual inductance. U.S. Pat. No. 5,682,098 describes such an MRI device that employs overlapping surface coils for this purpose. xe2x80x9cPlanar quadrature coil design using shielded-loop resonatorsxe2x80x9d, J. Magnetic Resonance, Vol. 125, pp. 84-91, A. Stensgaard (1997), describes another MRI device using a xe2x80x98dual modexe2x80x99 single surface coil for rotating field detection. Such designs work well in MRI where the nuclei are insensitive to RF fields in the direction of the polarizing static magnetic field, i.e. where the RF magnetic fields need only be orthogonal in two dimensions. However in NQR, parallel RF magnetic field components introduce a non-rotating field component that reduces the SNR improvement.
FIG. 3 illustrates a conventional surface coil array 40 for applying and detecting rotating RF magnetic fields that can be used in an NQR device, e.g. such as the one shown in FIG. 2. Coil array 40 includes a first coil 48 that is coplanar and co-centered (i.e. fully overlapping) with a second coil 49. More particularly, coils 48 and 49 as shown are what are commonly referred to as figure-8 coils, based on the geometrical shape and appearance. A xe2x80x9cfigure-8xe2x80x9d coil is understood to include any coil consisting of two coplanar sections enclosing an area. The sections are configured such that an external uniform RF magnetic field induces equal but opposite currents in the sections, the figure-8 coil thus performing like a receiving gradiometer. (By reciprocity, a coil may be considered to be either a transmitting or receiving coil.) The sections can therefore assume any geometric shape capable of this function and if desired can be spaced apart at a distance suitable for the particular application. Coils 48 and 49 are rotated 90xc2x0 with respect to one another, and with surface coil array 40 substituted for the birdcage coil 38 of probe 34, as switch 36 connects transmitter 30 to probe 34, coils 48 and 49 produce a magnetic field projecting orthogonal to their plane and thereby outside of probe 34. The combination of full overlap and 90xc2x0 rotation of the coils minimizes their mutual inductance so that the coils operate independently of each other. FIG. 4 shows that the RF magnetic fields generated by the coils 48 and 49 have significant parallel (or antiparallel) components perpendicular to the plane of the coils. For NMR applications these field components can be arranged along the static magnetic field direction where they do not contribute to the excitation or detection of the FID.
It would be desirable to provide an improved NQR apparatus having an increased sensitivity for detecting sub-kilogram amounts of target materials, and decreased sensitivity to acoustic and piezoelectric ringing, that is also capable of generating rotating RF fields outside the NQR device in order to expand the possible applications of its use.
According to the invention, an NQR apparatus for irradiating a sample with an RF magnetic field and for determining the presence of a target material by detecting an NQR signal includes a transmitter for generating an output signal at a selected frequency, a receiver for detecting the NQR signal, a probe, and a switch coupling the receiver and the transmitter with the probe for alternately connecting and disconnecting the receiver and the transmitter to the probe, thereby switching the NQR apparatus between a transmitting mode and a receiving mode. The probe consists of a resonant circuit that includes a coil array. The coil array includes a first figure-8 coil and a partially overlapping, second figure-8 coil for generating an RF magnetic field in a detecting region outside the probe. The first and second coils together alternate between a transmitting mode for irradiating a sample in the detecting region with an RF magnetic field, and a receiving mode for detecting an NQR signal indicative of the presence of a target material in the sample.
The invention also includes the method of using this apparatus whereby a sample is positioned within the detecting region outside the probe to detect an NQR signal indicating the presence of a quadrupole nuclei-containing target material in the sample.
The coil array disclosed here produces a more uniform rotating RF magnetic field than previous coil arrays. Increasing the uniformity of the rotating RF magnetic field increases the detection sensitivity and enhances the ability to separate magnetic resonance signals from piezoelectric and acoustic ringing. The coil array has applications in NQR detection of explosives and contraband, particularly landmine detection, personnel screening, and handheld screening of packages, baggage, and vehicles. The coil array may also find applications in magnetic resonance imaging.
Additional features and advantages of the present invention will be set forth in, or be apparent from, the detailed description of preferred embodiments which follows.